Silica particles, of micro- or macro- or nanometric size, are widely used as fillers in polymer matrixes in order to endow said polymer matrix with particular properties.
Several approaches have been used until now for introducing the silica particles into a polymer matrix.
The first approach consists of mixing the thermosetting polymer matrix, before its polymerization or crosslinking or hardening, with the silica particles by mechanical mixing, for example using a mixer.
However, this technique does not give a homogeneous dispersion of the silica particles in the polymer matrix, or does but with great difficulty.
The second approach was to use synthesis by the sol-gel route.
A first method according to this approach employs a silicon alkoxide such as tetraethoxysilane (TEOS) or tetramethoxysilane (TEMOS) in a solution of alcohol and water. This mixture is then heated in order to hydrolyze the silicon alkoxide. Then an acidic or basic catalyst is added to condense the silica in the form of particles.
Although studies have shown that, after thermal treatment, organic-inorganic composites can be formed by this technique, none has revealed the presence of well-defined silica nanoparticles. Moreover, the immiscibility of organic polymers, such as the epoxy or polyester-imide resins, with an aqueous solution and the increased reactivity of the monomers in acidic or basic conditions makes it difficult to prepare a composite of silica nanoparticles or polyester-imide/silica nanoparticle.
A second method is described in Mascia et al. “Selective Repartition of In-Situ Generated Silica Produced During the Evolution of an Epoxide Network from a Homogeneous Precursors Mixture and Effects on Properties”, Journal of Applied Polymer Science, Vol. 94, 1279-1290 (2004). In this method, silica particles are formed in situ in an epoxy-bisphenol A resin. This method comprises two steps: a step of obtaining a telechelic coupling agent, followed by a step of functionalization of the resin with said telechelic coupling agent. This method additionally requires the use of an acidic catalyst and leads to production of silica-perfluoroether microparticles during crosslinking of the polymer resin.
Matejka et al. describe a method in which silicon alkoxide is prehydrolyzed before being introduced into an epoxy-amine matrix. The mixture obtained is then crosslinked thermally to obtain an epoxy-silica hybrid polymer. A silica-epoxy interpenetrating network is thus formed. Although this synthesis is said to be in a single step, in which the components are added at the same time, hydrolysis of the alkoxide is first carried out in an acidic medium and then a change of pH is effected by adding polymeric base in excess. In particular, however, no well-defined silica particle is observed.
A third approach consists of making polymer matrix-silica nanoparticles composites starting from a bicontinuous phase system by functionalizing the polymer matrix, in a solvent or without solvent, and by using a coupling agent in order to create an epoxy-silica hybrid interphase and a silica network. But once again, formation of well-defined silica nanoparticles is not observed.
A fourth approach is described by Lee et al., “Nonaqueous Synthesis of Nanosilica in Epoxy Resin Matrix and Thermal Properties of Their Cured Nanocomposites”, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 755-768 (2006). This method consists of carrying out silica synthesis in a nonaqueous medium using an acid salt (BF3MEA) as catalyst of TEOS. This salt allows silica nanoparticles to form in the epoxy resin (DGEBA). However, this technique generates ions, which is incompatible with application in the medium and high voltage electrical industry.
Finally, it has been proposed to introduce silica nanoparticles into a polymer matrix using a colloidal solution of silica. The colloidal solution of silica is mixed with the resin by mechanical means before polymerization.
Thus, in the methods for synthesis of a polymer matrix doped with silica nanoparticles, either the silica nanoparticles synthesized are poorly dispersed in the polymer matrix, or the method does not allow the synthesis of well-defined silica nanoparticles of controlled size. Moreover, the methods of synthesis of the prior art are multistep methods that use solvents and coupling agents, or else require functionalization of the polymer. In all these methods, synthesis of silica nanoparticles takes place during crosslinking of the polymer and not before.